feat: add daily digest email notification module

- New DailyDigestModule with scheduled summary email for open dashboard items
- Extract StatsService from StatsController for reuse in digest
- Add DailyDigestEnabled, UserEmail, UserPreferredUsername to UserSettings entity
- Sync email/username from OIDC token on each get/update call
- Add dailyDigestEnabled to UserSettingsDto and update API
- Notifications tab in UserSettingsPage with enable toggle and "Jetzt senden" button

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

paperless-backend/src/stats/stats.module.ts

@@ -1,6 +1,7 @@
 import { Module } from '@nestjs/common';
 import { TypeOrmModule } from '@nestjs/typeorm';
 import { StatsController } from './stats.controller';
+import { StatsService } from './stats.service';
 import { Email } from '../database/entities/email.entity';
 import { InboxModule } from '../inbox/inbox.module';
 import { PaperlessModule } from '../paperless/paperless.module';
@@ -12,5 +13,7 @@ import { PaperlessModule } from '../paperless/paperless.module';
     PaperlessModule,
   ],
   controllers: [StatsController],
+  providers: [StatsService],
+  exports: [StatsService],
 })
 export class StatsModule {}